1. Field of the Invention
The present invention relates to an optical transceiving system and an optical receiving apparatus.
2. Description of the Related Art
At present, there have been proposed a variety of technologies for increasing the reliability of transmission links in the field of optical communications. For example, a technology has been proposed for operating a plurality of frame synchronizing circuits parallel to each other and establishing frame synchronization with any one of the frame synchronizing circuits (see, for example, JP11-331140A). According to this proposed technology, however, it is difficult to see if the transmission data contain many errors.
FEC (Forward Error Correction) processes with a high error correction capability have been made practically feasible by the advances of digital circuit technologies. Therefore, attention has been paid to a technology for increasing the reliability of transmission links by correcting errors based on FEC processes.
FIG. 1 of the accompanying drawings is a block diagram showing the configuration of an optical transceiving system incorporating an FEC process therein. As shown in FIG. 1, the optical transceiving system includes optical transmitting apparatus 70 and optical receiving apparatus 80 which are connected to each other by optical fiber transmission link 40. Optical transmitting apparatus 70 includes frame generating circuit 71, FEC encoder 72, and electrooptical converting circuit 75. Optical receiving apparatus 80 includes frame terminating circuit 81, FEC decoder 82, optoelectrical converting circuit 85, synchronizing word detecting circuit 91, frame synchronization detecting circuit 92, and receiver frame synchronization display circuit 95.
FIG. 2 of the accompanying drawings is a diagram showing by way of example the makeup of a frame of an optical system that is used in a general OTN (Optical Transport Network) including the optical transceiving system shown in FIG. 1. As shown in FIG. 2, one frame is made up of 16,320 bytes including 6 bytes at its beginning, with a synchronizing word for establishing frame synchronization being assigned thereto.
FIG. 3 of the accompanying drawings is a timing chart illustrative of the operation of the optical transceiving system shown in FIG. 1.
Synchronizing word detecting circuit 91 of optical receiving apparatus 80 receives a frame generated by frame generating circuit 71 of optical transmitting apparatus 70, and detects the synchronizing word of the frame. Frame synchronization detecting circuit 92 determines whether or not the detected synchronizing word contains a bit error. If frame synchronization detecting circuit 92 detects a plurality of successive synchronizing words (hereinafter, two synchronizing words) free of a bit error, then receiver frame synchronization display circuit 95 makes a receiver frame synchronization display signal high, thereby establishing frame synchronization.
As shown in FIG. 3, even though the synchronizing word of a frame having frame number #3 is free of a bit error, if the synchronizing word of a frame having frame number #4 suffers a bit error, then no frame synchronization is established.
As the error rate of optical fiber transmission link 40 increases, the probability that synchronizing words contain bit errors becomes higher, making it time-consuming to establish frame synchronization. Consequently, many data signals are discarded, and the throughput of the communication network is reduced.